Building Materials, Third Edition

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s ‚™ also known as primary, unstratified or eruptive rocks are of volcanic origin and
are formed as a result of solidification of molten mass lying below or above the earth’s surface.
The inner layers of the earth are at a very high temperature causing the masses of silicates to
melt. This molten mass called magma is forced up as volcanic eruptions and spreads over the
surface of earth where it solidifies forming basalt and trap. These are known as effusive rocks.
If the magma solidifies below the earth’s surface itself, the solid crystalline rock is termed as
deep-seated plutonic rock. The examples are granite, syenite, diorite and gabbro. If the magma
solidifies at a relatively shallow depth, the resultant rock possesses a finely grained crystalline
structure—and is termed as hypabyssal. Dolerite is such a rock. The principal constituents of
magma are quartz, mica and felspar. The texture of the rock is greatly influenced by the rate of
cooling of the magma. The classification of igneous rocks on the basis of silica content is as in
Table 3.2.

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— ‚™ are also known as aqueous or stratified rocks. The various weathering
agencies, e.g. rain, sun, air, frost, etc. break up the surface of earth. Rain water carries down
these broken pieces to the rivers. As the rivers descend down to the plains, the velocity
decreases gradually and the sediments (disintegrated rock pieces, sand, silt, clay, debris, etc.)
in the water settle. Due to the seasonal variation, sedimentation takes place in layers. With
time, the sediments get consolidated in horizontal beds due to the pressure exerted by overlying
material.
The properties of the sedimentary rocks vary considerably depending upon the nature of the
sediment and type of bond between the sediment and grains. Usually, the rocks are well
stratified and show well defined bedding planes. The rocks are soft and can be easily split up
along the bedding as well as normal planes. The examples of sedimentary rocks resulting from
the precipitation of salts in drying water basin (chemical deposits) are gypsum, anhydrite,
magnesite, dolomite, lime tufas. Sedimentary rocks resulting from the accumulation of plant or
animal remains (organogenous rocks) are limestone, shale, chalk, diatomite and tripoli. The
examples of rocks resulting from the deterioration of massive magmatic or sedimentary rocks
(fragmental rocks) are sandstone, sand, gravel, carbonate conglomerate and breccia.